The present invention relates to an igniter for electric ignition systems used for electric detonators and automotive air bag gas generators.
The conventional type of igniter for electric ignition systems used for the above-mentioned applications is made up of two lead wires 11, with their ends functioning as a pair of terminals 13, and a heating element 15 disposed between the paired terminals 13, as shown in FIG. 8. The heating element 15 is a fine platinum wire or nichrome wire having a high electric resistance, and it is bonded and connected to the terminals 13 with solder or electrically conductive adhesive 17. When turned on, this type of igniter for electric ignition systems 18 permits an electric current to flow through the high-resistance heating element 15 via the conductors 11 of the lead wires and the terminals 13, generating heat in the heating element 15 as the result of conversion from electric energy into thermal energy.
The above-mentioned conventional type of igniter for electric ignition systems 18 may be used for an electric detonator (electric ignition system) as shown in FIG. 9, in which the reference numeral 19 denotes two lead wires to supply an electric current. Between the ends of the conductors 11 of the lead wires 19 is disposed the igniter for electric ignition systems 18. Around the igniter for electric ignition systems 18 is disposed the priming charge 21. On the priming charge 21 is disposed the blasting powder 23. The lead wires 19 and the priming charge 21 are surrounded by the insulation resin 25 and 26, respectively, which prevent the priming charge 21 from being ignited by static electricity. When triggered, the electric detonator mentioned above permits an electric current to flow through the lead wires 19 and the igniter for electric ignition systems 18. The electric current generates thermal energy which ignites the priming charge 21 which, in turn, explodes the blasting powder 23.
In the meantime, as the speed of automobiles increases, the development of the safety air bag is going on for the reduction of shocks to the driver in the event of an accident. The safety air bag is inflated by a pyrotechnic gas generator. The igniter 18 for the gas generator is required to have a high reliability so that it never works when the car is in the normal state but generates a gas instantaneously for the reduction of shocks to the driver in the event of an accident. In the conventional safety air bag, this reliability is achieved by providing the igniter 18 with two or three heating elements 15 arranged in parallel so that ignition takes place certainly even in the case where one of the heating elements 15 is disconnected from the terminal 13.
The conventional ignitier 18 mentioned above has a disadvantage that there is no way to confirm that the fine heating element 15 is firmly connected to the terminal 13 of the lead wire 19 because the connection is made with adhesive or solder. It has another disadvantage that there is a possibility that the heating element 15 is disconnected from the terminal 13 while the air bag is left unused for a long period of time in an environment where there are considerable temperature changes, vibrations, and shocks. With the heating element disconnected, the igniter does not work.
For the conventional igniter 18 to be highly reliable, it is necessary that the igniter 18 be provided with two or three heating elements arranged in parallel. This creates difficulties of making the fine heating elements uniform in resistance and igniting performance.
The present invention was completed to solve the above-mentioned problems. Accordingly, it is an object of the present invention to provide an igniter for electric ignition systems which maintains its high reliability even in a severe environment.
The gist of the present invention resides in an igniter for electric ignition systems which comprises a pair of terminals of metal thin film formed and disposed a certain distance apart on the surface of a thin insulating board and at least one heating element of metal thin film which integrally connects said terminals to each other.
The gist of the present invention resides also in an igniter for electric ignition systems which comprises a pair of terminals of metal thin film formed and disposed a certain distance apart on the surface of a thin insulating board and at least one heating element of metal thin film which integrally connects said terminals to each other, said heating element being covered with a protective coating film of inorganic thin film.
According to the present invention, the terminals and heating elements are integrally made of metal thin film. This structure permits an electric current to flow through the terminals and heating element certainly and to generate heat in a short time because of the small mass of the heating element.
According to the present invention, the heating element is covered with a protective coating film made of an inorganic material to ensure good heat conduction and to maintain the initial performance over a long period of time.